This invention relates to a method for conversion from a field signal to a frame signal, and more particularly to an improved method of the kind above described by which a frame signal obtained by interlaced scanning can be reproduced with a circuit of simple structure.
In a method for recording a video signal on a recording medium such as a magnetic tape or a magnetic disk, one track is allotted to one field or two fields so that video signal portions are recorded in side-by-side relation with the frame period or field period, in order to facilitate reproduction of a special picture such as a still picture or a slow-motion picture. In the case where one track is allotted to one field, a method is most frequently employed in which, in the record mode, an odd-field signal portion only of a composite video signal including synchronizing signals is recorded on a track and, in the playback mode, the same track is scanned twice to obtain or reproduce the composite video signal portion corresponding to one frame. That is, conversion from a composite video signal portion corresponding to one field (referred to hereinafter as a field signal) to a composite video signal portion corresponding to one frame (referred to hereinafter as a frame signal) is attained by the method above described. The principal purpose of this method is to improve the recording density of the video signal on the recording medium.
In order to realize the effect of interlaced scanning by such a method used for conversion from a field signal to a frame signal, it is not sufficient to merely repeatedly reproduce the field signal twice, and a special contrivance must be made. This is because, while a time lag of a 1/2 horizontal scanning period (referred to hereinater as 1/2H) is required between an odd-field video signal and an even-field video signal, it is also required to maintain a time interval of one vertical scanning period (referred to hereinafter as 1 V) between their vertical synchronizing signals.
Therefore, according to a prior art conversion method of this kind, an even-field signal or an odd-field signal delayed by 1/2H relative to the other and the odd-field signal or the even-field signal not subjected to the delay are alternately selected by changing over a switch, and a synchronizing signal generator is separately provided to produce a signal portion including the synchronizing signals. In this manner, the field signals delayed and not delayed are selected at the time interval of 1 V by the switch for a predetermined period of time during transition from one field to the next. Thus, a frame signal is obtained in which the video signals of the adjacent fields have a relative delay of 0.5H, and the vertical synchronizing signals appear periodically at a time interval of 1 V.
However, in view of the fact that the field signal delayed by 1/2H, the signal portion including the synchronizing signals, and the field signal not delayed and remaining intact are sequentially repeatedly selected by the switch in the prior art conversion method of this kind, the prior art method is defective in that a complex and troublesome procedure is required for the control of the switch, and the necessity for separate provision of the synchronizing signal generator leads necessarily to the complexity of the structure of the circuit.